It is known that a light beam can be deflected by the interaction with ultrasound waves inside a solid material. This phenomenon is called acousto-optic deflection. FIG. 1 depicts an acousto-optic modulator or scanner 1. The solid medium of this scanner can be a glassy material. In most technical applications, however, it is a crystal material. A radio-frequency signal is fed to a piezoelectric sound transducer (4) attached to the surface of the crystal (1). An ultrasonic beam (3) is launched in the crystal. The entering light beam (5) propagates through the transparent crystal material and exits partly as an undeflected light beam (6), partly as a deflected light beam (7). The deflection is produced by diffraction of the light at the lattice, which is deformed by the ultrasonic wave. With proper choice of the input angle H.sub.o and the radio frequency most of the optical power I.sub.i of the input beam is coupled to the diffracted beam 7 with angle H.sub.-1 and optical power I.sub.i. The indices stand for the so-called diffraction order. With the geometry shown, almost all of the input light is coupled to the diffraction -1 [minus one]. The optical power I.sub.0 of the undeflected beam 6 is strongly attenuated.
The deflection follows the laws of optics and the angle of the deflected beam is controlled only by the relative wavelengths of light and ultrasound waves inside the crystal. This provides very precisely controlled deflection angles. An increase in the radio frequency gives a shorter sound wavelength and a deflected beam 8 with a larger deflection angle, as indicated by the broken line in FIG. 1. Since the frequency of a signal can be very accurately controlled, the deflection angle is also very accurately controlled.
Of special interest are those applications in which the radio frequency increases linearly with time. This results in deflection angles which grow in almost linear fashion with time over a given angle range. This deflection angle range is typically one to two degrees. An acousto-optic deflector operated in this way can be used in conjunction with focusing optics, so that a focus spot is created on the surface. The spot scans the surface with high speed and great precision, applying reading or writing information to the surface.
A well-known phenomenon in linear scanning is a cylinder-lens effect created by the scanner, as shown in FIG. 2. With an increasing ultrasound frequency the sound wave 9 that most recently left the transmitter has a higher frequency than the sound wave 10 that is further removed from the transducer 2. Thus the deflection angle of a beam segment 11 closer to the transducer is larger than the deflection angle of a beam segment 12 further away from the transducer 2. The magnitude of this effect depends on the scanning speed and can be compensated for by an external cylinder lens 13, with the result that a parallel beam 14 leaves the lens.
An acousto-optic deflector can be simply depicted, as in FIG. 1. Employed is an optically isotropic medium like dense flint glass. One of the deflector's important parameters is the useful scanning-angle range, often expressed as the useful band width of the coupled RF signal. Coupling to the deflected beam is efficient within this range. The undeflected beam Io is strongly attenuated. Outside of the useful range the coupling is weaker and the beam, though deflected by means of diffraction, retains only a small portion of the input optical power, i.e. I.sub.-1 &lt;&lt;I.sub.i. The coupling efficiency as a function of angle is shown in curve 15 of FIG. 3 for a simple deflector with an isotropic medium.
From the IEEE Transactions on Sonics and Ultrasonics, Vol. SU-23, No. 1, pages 2-22 (1976), I. C. Chang, "Acousto-optic Devices and Applications" it is known that the optical anisotropy of certain crystals can be used to increase the useful angle range, as is shown in curve 16 of FIG. 3. A frequently used medium is paratellurite (TeO.sub.2), which is strongly anisotropic. This publication also discloses a method to further enlarge the useful range of deflection by cutting the optically anisotropic crystal at an angle to the optical axis 18, FIG. 4. This results in an enlarged efficiency curve 17 (FIG. 3) for the angle range.
Also known for such off-axis devices is a phenomenon called acoustical walk-off. For some crystals the velocity of sound in a given direction is greater than in a direction perpendicular thereto. In TeO.sub.2 this effect is very highly pronounced. The sound velocity is 617 m/s in the [1,1,0] crystal direction and 4200 m/s in the [0,0,1] crystal direction. When the piezoelectrical transducer is mounted at a slight angle to the crystal axis, the beam propagates in both of these directions. Propagation is significantly faster in the one direction, however. The result is that the sound wave's direction of propagation is not perpendicular to the transducer, but instead runs in a direction 18 more inclined towards the fast axis. The angle to the normal direction of the transducer is called the walk-off angle. In TeO.sub.2 the walk-off direction can be 45.degree. or more. The wave planes always run parallel to the transducer, i.e., the walk-off phenomenon does not change the angle of the input beam to be chosen for best efficiency. It takes effect only at that place within the crystal where there is interaction between acoustic wave and lightwave.
If the acousto-optic scanning procedure is operated close to its limits by increasing the usable angle range and increasing the scanning speed, wavefront errors in the deflected beam will result that make it impossible to get a good focus spot across the entire scan. The primary errors are curvature of the best focus plane and coma. The result is deteriorated contrast and definition in the direction along the scan.